An automotive antitheft system has been put to practical use, so that a vehicle is protected against theft which could be otherwise carried out by break-in, breakage of window glass, or jack-up, and so on. Such a conventional antitheft system is known in the art, for example, as disclosed in Japanese Patent Publication No. 2003-34233. An automotive antitheft system is practically used to protect the vehicle against the theft by the jack-up, wherein an inclination sensor is provided such that a detecting axis of the inclination sensor (an acceleration sensor) is arranged in parallel to the ground surface in order to detect an inclination of the vehicle by detecting gravitational acceleration.
Recently, demand for the automotive antitheft system having the inclination sensor is increasing. In some cases for the insurance against the theft, the automotive antitheft system is necessary to be installed in the vehicle.
However, various kinds of components and parts are necessary to be mounted in the vehicle, when the antitheft system having the inclination sensor is installed. Accordingly, a mounting space for such system becomes necessary, in addition to the increase of a business investment.
An inclination sensor to be mounted in an automotive vehicle is known in the art, for example, as disclosed in Japanese Patent No. 3622723. According to such a conventional inclination sensor, two detecting elements are provided in the vehicle, such that each of the detecting elements detects the respective inclinations of the vehicle in a longitudinal direction and a cross direction of the vehicle. As a result, the inclination sensor detects the inclination of the vehicle.
For example, as shown in FIGS. 30A and 30B, the inclination sensor 310 is mounted in an automotive vehicle 300, such that a detecting direction GF of one of the detecting elements is a longitudinal (forward and backward) direction (F-B direction), whereas a detecting direction GW of the other detecting element is a cross (left and right) direction (L-R direction) of the vehicle 300. In FIGS. 30A and 30B, letters F, B, L and R respectively designate the directions of the front, the back, the left, and the right of the vehicle. An acceleration sensor is used for the detecting elements.
In the above inclination sensor, an average amount of the acceleration in the F-B direction during a predetermined period is memorized as a reference value GFi, when the vehicle is parked in a normal condition. And an average amount of the acceleration in the L-R direction during the same predetermined period is likewise memorized as a reference value GWi. And current accelerations GF1 and GW1 in the F-B and L-R directions, which are periodically detected by the sensor, are respectively compared with the reference values GFi and GWi, and the inclined angle of the vehicle 300 is calculated by differences between the current accelerations and the reference values.
Since the differences between the current accelerations and the reference values, respectively correspond to horizontal components of the gravitational accelerations in case of the inclination of the vehicle, the inclined angle of the vehicle 300 can be calculated by such differences of the accelerations.
In the above conventional inclination sensor 310, however, the detecting elements detect the accelerations in the longitudinal (F-B) and cross (L-R) directions, individually, as mentioned above. The inclination of the vehicle 300 in the longitudinal direction, which occurs at a braking or accelerating the vehicle or when the vehicle is stolen by use of a tow truck, or the inclination of the vehicle in the cross direction, which occurs at turning the vehicle can be detected by only one of the detecting elements.
Accordingly, in the case that one of the detecting elements (e.g. the detecting element for the F-B direction) is deteriorated, the inclination of the vehicle can be detected by the other detecting element (detecting element for L-R direction) only in the cross direction. Therefore, it is a problem in the above case that detection accuracy is remarkably decreased.
Another antitheft system for a vehicle is disclosed, for example, in Japanese Patent No. 3622723, according to which various conditions of the vehicle are detected based on output from acceleration sensors to prevent a vehicle theft.
More exactly, the antitheft system of the above prior art has an acceleration detecting means for detecting accelerations in multiple directions of the vehicle, and a determining means for determining an abnormal condition to be caused by a possible vehicle theft based on the accelerations in the multiple directions. The determining means calculates offset values for the longitudinal and cross directions of the vehicle, wherein the offset values are obtained as average values of accelerations of the vehicle, which is in a parking (stable) condition, for a predetermined period. The determining means calculates difference values between the offset values and current accelerations, which are periodically detected, as effective accelerations. When the effective accelerations become higher than an allowable error, an integration for the effective accelerations is performed. Then, the determining means determines whether an integrated value exceeds a predetermined reference value, so that it can detect whether the vehicle has been moved or inclined.
In the above antitheft apparatus for the vehicle, however, it is not possible to detect the possible vehicle theft, when the vehicle is jacked up with such a small acceleration not exceeding the allowable error and carried away with such small acceleration. When the allowable error is made to be small, even a small swing of the vehicle caused by wind may be erroneously detected as the vehicle theft.